August 2018 Issue

Foreign Service

By Dan Marinucci

Diagnostic trouble codes signal when a system or component is not functioning properly, often in very precise terms. So why not begin your diagnosis at the DTC’s source?

Careful testing at the indicated source of a trouble code usually is the shortest path to a successful diagnosis. However, some technicians stray from this path because they’re distracted or undisciplined. As this saga shows, the result may be heartburn and wasted time.

“Dapper Dave” is an old pal who operates a general repair shop in a rural town. He and his lead tech “Big Bryan” faithfully try to commit the resources necessary to fix vehicles correctly the first time. This approach has generated word-of-mouth advertising that strengthened their business.

Nonetheless, distractions and disruptions characterized the great gift-car gamble. You see, a regular customer impulsively donated his damaged Mercedes to the shop. The car was a 2002 C230 equipped with a 2.3L engine and automatic transmission. The fellow had invested in regular maintenance but also drove the car very hard; a head gasket failed at 160,000 miles. Dave and Bryan decided to repair the vehicle because they thought it would make a distinctive courtesy car for the shop.

They purchased a low-mileage Mercedes engine from a reliable salvage yard and installed it when the shop schedule finally slowed down. The engine swap went smoothly. However, DTC P0335— for the crankshaft position sensor (CKP)—appeared after the boneyard engine started up. In the past, Dave and Bryan had road-tested this car enough to know how it should perform. Now it accelerated very sluggishly. The engine cranked over okay but took a long time to start. Worse yet, occasionally it wouldn’t start at all. Then, inexplicably, it would restart after repeated tries. The engine sometimes stalled at an intersection or during light-throttle driving at cruising speeds. However, it did restart promptly.

The next time he had a chance to work on the Mercedes, Dave tried a shortcut: He replaced the CKP sensor only. But this didn’t eliminate the DTC or the car’s symptoms. Then one of Dave’s buddies convinced him to replace the CKP sensor harness because this repair had fixed a C230 with the same symptoms at his shop. Bryan personally retrieved a good-looking harness from a salvage-yard C230 and patiently installed it in the gift car. This CKP harness ran from the sensor itself over to the signal acquisition module (SAM). But this made no improvement in the car.

By now, Dave’s patience with the car was wearing thin. So he subcontracted the diagnosis to a European car specialist in a neighboring town. The specialist accomplished nothing despite charging $600 to check the Mercedes and replace its SAM module. What’s more, he urged Dave to let him know what, if anything, ever fixed the car.

Eventually, frustration with the car prompted two decisions. First, Dave and Bryan teamed up and worked on the car uninterrupted. Second, Bryan planned to check the CKP sensor with an oscilloscope. Admittedly, they could have and should have done this when the DTC first occurred. Anyway, the sensor’s two wires went directly to the SAM, which sat next to an underhood fuse panel in the left rear corner of the engine compartment. Simply unsnapping the fuse panel cover gave them easy access to the CKP wires. Coincidentally, the camshaft (CMP) sensor wires also were readily accessible because the sensor was on the top of the engine.

Both Dave and Bryan were still acclimating themselves to a Pico scope system they had installed on the shop’s laptop computer. However, it took only minor tinkering with the voltage scale and time base to get the patterns shown in the cell phone photo at right. Bryan wasn’t completely comfortable yet with the save and download functions on the scope system, so he took a picture of the patterns on the laptop screen with his cell phone for me. The CKP signal is the blue trace; the CMP signal is the red trace. The blue trace is an analog signal while the red one is a digital signal.

Next, they gathered up all available clues to this Mercedes mystery. First and foremost, something disturbed the CKP signal severely enough to set a DTC right away. Second, this code (P0335) remained no matter how long someone drove the car. Third, the scope pattern proved that the sensor was generating a signal and the signal reached the SAM module. Therefore, the sensor, connections and wiring might very well be okay after all.

Bryan believed he had good reason to suspect that the CKP signal was flawed. Fortunately, he already had been practicing scope tests on several healthy Toyota CKP sensors. To him, this Mercedes CKP pattern strongly resembled some Toyota signals—except for the fact that ragged, unusual gaps kept recurring throughout the pattern. Maybe the component triggering this CKP sensor could not trigger it consistently for some reason.

Some additional homework revealed that this “trigger component” was a ring of steel teeth or stubs mounted on the flywheel. Furthermore, removing a rubber plug from an inspection hole on the side of the engine enabled Dave and Bryan to visually inspect the trigger teeth. Ouch! Two of the trigger teeth were badly bent! For your information, this inspection hole is on the passenger side of the engine where the transmission bolts to it. The hole provides access to the torque converter bolts.

Dave and Bryan theorized that the salvage guys may have been jockeying the engine around with pry bars—positioning it on a shipping pallet. Maybe someone was a bit too careless with a pry bar and bent those triggering teeth. Anyway, these guys were not easily deterred. Dave tried to straighten those teeth by sliding a pry bar into the engine’s inspection hole. This shortcut backfired because he ended up bending the teeth worse than they already were. In fact, he bent the teeth so badly that they scraped the tip of the CKP sensor and destroyed it!

Finally, installing the flywheel from the original engine and replacing the dinged CKP sensor fixed the Mercedes for good. They were very lucky that the local junk man had not picked up the original engine yet. Otherwise, they would have had to buy a flywheel, too. We can never have too much good luck, agreed?

Hindsight is always 20/20. But testing at the potential source of the DTC—the CKP sensor itself—turned out to be both easy and informative. Subconsciously, perhaps, Dave and Bryan talked themselves out of a relatively fast, simple diagnosis. Rest assured that they’ll know better next time. What’s more, they’ll always inspect a used engine much more thoroughly—especially both sides of the flywheel.